Unless it breaks or starts sounding like it is "clunking", I don't.

For piecing, I generally change it with each new project. I don't do a ton of piecing, though -- definitely way less than 10 hours per quilt. (I've always heard to change every 8 hours -- go figure)
With quilting, I use a thick cotton batting (Warm & Plush) that dictates the necessity of a very sharp needle so I'm changing my needle fairly often with that. Definitely at least every 6 hours, sooner if my stitches aren't forming as nicely or I see it hesitating before pushing through the quilt.

Only if there is a burr, breaks or seems dull.

I change my needle when it becomes dull, gets a burr, or bends. Or is just not the right needle for the thread or project I am working with...so, as needed.

I don't really have a time frame, but I definitely change the needle prior to quilting (or FMQ) a top on my domestic machine. This saves so many tension issues. As an added note, I always fill 3 bobbins before I start the quilting process. I end up changing my needles a lot.

I have heard 8 hours. I have also heard when you refill the bobbin.

I think it also depends on the wear & tear on your needle.

That's what I do, too.

If I take a needle out to change it to another size or type, I don't keep the needle. Keeping track of used needles would be a hassle for me.

I pay attention to my needle, it usually tells me when it needs changing.

I love your idea of keeping track of needle sizes.I 'm going to have to copy this idea,if you don't mind!Occasionally I'll use a needle for a short time for a specific purpose and don't want to put it back in with the brand new ones,when I change them out.